Various magnetic fluid compositions have hitherto been proposed for use in magnetic fluid-sealed apparatus, including a dispersion comprising a carrier having stably dispersed therein ferromagnetic fine particles with the aid of a surface active agent carrying a lipophilic group having a similar structure to that of the carrier as disclosed in JP-A-64-27207 (which corresponds to U.S. Pat. No. 5,085,789) (the term "JP-A" as used herein means an "unexamined published Japanese patent application").
A magnetic fluid-sealed apparatus is generally composed of an axis, a housing affording a cylindrical space surrounding the axis, and a magnetic circuit-forming member fitted into the cylindrical space which is composed of a pair of ring pole pieces with a ring permanent magnet being interposed therebetween. A magnetic circuit is formed in such a manner that the magnetic flux flows from the permanent magnet into one of the pole pieces, goes to the other pole piece via the axis, and returns to the permanent magnet. A magnetic fluid composition injected into the ring gap between the periphery of the axis and the inner periphery of the pole pieces is held there by the magnetic force of the magnetic circuit and functions as a magnetic seal.
Known magnetic fluid-sealed apparatus designed to prevent splash of the magnetic fluid seal include the one disclosed in JP-A-3-163271. The disclosed magnetic fluid-sealed apparatus is characterized in that a second permanent magnet is provided between one of the pole pieces and a roller bearing to increase the magnetic flux toward one of the pole pieces and thereby control the excess of the magnetic fluid composition held in the gap between the periphery of the axis and the inner periphery of the other pole piece thereby to prevent splash.
Further, a magnetic fluid composition having improved anti-splash properties and a magnetic fluid-sealed apparatus using the same is disclosed in Japanese Patent Application No. Hei-4-55443, in which a low-volatile organic solvent constituting the magnetic fluid composition as a carrier has an increased viscosity so that the viscosity of the magnetic fluid composition may fall within a certain range.
However, where the above-mentioned conventional magnetic fluid-sealed apparatus is operated by rotating the housing at such a high speed that the rotational speed at the sealing part reaches 2 m/sec or even higher, the magnetic fluid composition applied in an amount necessary for forming a ring seal cannot be sufficiently held due to the high centrifugal force, resulting in splash of the fluid. It has therefore been demanded to further improve anti-splash properties of a magnetic fluid composition to be used in a magnetic fluid-sealed apparatus.
On the other hand, the ferromagnetic fine particles dispersed in a carrier with the aid of a surface active agent tend to aggregate and be localized in the composition with time, which also makes the composition liable to splash.
Splash of a magnetic fluid composition causes dust of the ferromagnetic fine particles or shortage of the magnetic fluid composition in the sealing part and a reduction in pressure resistance of the sealing part, resulting in the failure of functioning as a dust seal.